1. Field of the Invention
The present invention relates to a negative electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a method for producing a negative electrode for a lithium ion secondary battery.
2. Related Art
In recent years, portable devices such as cellular phones, laptop computers and camera integrated VTRs form a large market. A lightweight and portable secondary battery having high energy density is strongly required as a power supply used in these portable devices. In particular, a lithium ion secondary battery has a comparative advantage in these required characteristics as compared with a secondary battery, and is progressively employed for portable devices. In the lithium ion secondary battery, upon discharging, lithium existing in a negative electrode is oxidized and lithium ions thus formed are released, while lithium ions existing in a positive electrode are reduced and a lithium compound thus formed is occluded. Upon charging, lithium ions in a negative electrode are reduced and lithium thus formed is occluded, while a lithium compound existing in a positive electrode is oxidized and lithium ions thus formed are released. As mentioned above, in the lithium ion secondary battery, upon charging/discharging, lithium ions move between the positive electrode and the negative electrode, and thus occluded as lithium or a lithium compound in any one of electrodes.
Electricity consumption of the portable device may increase more and more, and charge/discharge of the secondary battery is frequently repeated. In addition to required characteristics, an improvement in charge/discharge cycle characteristics, life extension and rapid charging are required to the lithium ion secondary battery, and thus an improvement in each member composing the lithium ion secondary battery is in progress.
A metallic material, which enables a large capacity by combining with lithium to form an alloy, has attracted attention as a negative electrode active material used in a lithium ion secondary battery. The metal, which combines with lithium to form an alloy, includes tin or an alloy containing tin, lead, silver and the like, and tin or a tin alloy is suited for practical use in view of material costs.
The present inventors propose, as a negative electrode base material which enables the production of a lithium ion secondary battery having high output voltage and high energy density, a negative electrode base material in which a tin film is formed on a patterned organic film (see Patent Documents 1 to 3). In these Patent Documents, the output voltage and the energy density are enhanced by increasing a surface area of the tin film. However, when the present inventors have evaluated charge/discharge cycle characteristics by lithium ion secondary batteries using the negative electrode base material of Patent Documents 1 to 3, it has been found that there is a problem. The problem is that alloying of lithium and tin, which occurs upon charging, is likely to occur concentratedly at the pillar-shaped tip portion and volume expansion involved in alloying becomes likely to occur at the tip portion, leading to the increased tip portion, and thus resulting in cracking at the root of a pillar. It has also been found that there is a problem that a tin film formed on a patterned organic film is peeled in the case of repeating charging/discharging.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2009-123380
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2009-123379
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2008-251475